This invention relates to a thermally conductive molded article that has excellent thermal conductivity and a method of making the same. More particularly, this invention relates to a thermally conductive molded article suitable for use as a heat radiation member, heat transfer member, or a component thereof, each of which effectively dissipates the heat from electronic parts such as semiconductor element, a power supply, and a light source in electronic hardware outside; and a method of making the same.
With recent advancements, miniaturization, and lightening of electronic hardware, semiconductor packages have become more compact and more highly integrated and operated at higher speed. Therefore, the heat generated by the electronic hardware is a very important issue. Generally, to dissipate the heat from heat-generating components to outside, a sheet material and polymer grease are placed between a radiator and one of the followings: a printed circuit board; a semiconductor package; and a heat radiator such as a radiation plate or a heat sink. The sheet material and the polymer grease are generally made of a thermally conductive polymer composition or a thermally conductive molded article.
Such a thermally conductive composition and thermally conductive molded article includes a matrix, such as resin and rubber, and filler that has high thermal conductivity in the matrix. Possible fillers include metal oxide, metal nitride, metal carbide, and metal hydroxide. Examples of such possible fillers include aluminum oxide, boron nitride, silicon nitride, magnesium oxide, zinc oxide, silicon carbide, quartz, and aluminum hydroxide.
Thermally conductive compositions and thermally conductive molded articles that include graphite powders or carbon fibers as filler are known. For example, Japanese Laid-Open Patent Publication No.62-131033 discloses a molded body made of thermally conductive resin in which the resin is filled with graphite powders. Japanese Laid-Open Patent Publication No.4-246456 discloses a composition of polyester resin containing carbon black or graphite. Japanese Laid-Open Patent Publication No.5-17593 discloses a thermally conductive molded body of great mechanical strength in which the carbon fibers are arranged in a certain direction and are impregnated with graphite powder and thermosetting resin. Japanese Laid-Open Patent Publication No.5-222620 discloses a thermally conductive material using pitch-based carbon fibers that have a specific cross section. Japanese Laid-Open Patent Publication No.5-247268 discloses a rubber composition in which is mixed synthetic graphite having a particle size of 1 to 20 μm. Japanese Laid-Open Patent Publication No.9-283955 discloses a thermally conductive sheet in which the graphitized carbon fibers of specific aspect ratio are dispersed in polymer, such as silicone rubber. Japanese Laid-Open Patent Publication No.10-298433 discloses a composition and a thermally conductive sheet in which silicone rubber has, mixed within it, spherical graphite powders having an interplanar spacing of crystals from 0.330 to 0.340 nm. Japanese Unexamined Patent Publication No. 11-158378 discloses a silicone rubber composition that has electrical conductivity and thermal conductivity and in which heated graphite particles are mixed. Japanese Unexamined Patent Publication No. 11-279406 discloses a composition that has good electrical conductivity and thermal conductivity and in which carbon fibers of a given length are mixed in silicone rubber.
As for carbon material that is produced by graphitizing polymer by heating, graphite films that are produced by graphitizing polymeric film by heating to have excellent thermal conductivity and methods of making the graphite films have been widely proposed, as disclosed in Japanese Laid-Open Patent Publication No.3-75211, Japanese Laid-Open Patent Publication No.4-149012, Japanese Laid-Open Patent Publication No.9-156913, and Japanese Laid-Open Patent Publication No.2000-178016. Meanwhile, Japanese Laid-Open Patent Publication No.9-324127 discloses highly thermally conductive powdery graphite, die bond adhesive for a semiconductor element, and a semiconductor device in which the powdery graphite is blended in adhesive thermosetting resin. The powdery graphite is obtained by graphitizing a polymeric film by heating and pulverizing or cutting the resultant graphitized film.
However, for recent high performance electronic parts, due to increase in amount of heat generation, the need for greater thermal conductivity has increased. Therefore, the thermal conductivity is still insufficient for the above-mentioned molded articles that include conventional graphite powder or carbon fibers as thermally conductive filler.
In addition, in relation with miniaturization and lightening of electronic parts, a thermally conductive molded article that has an excellent thermal conductivity in a certain direction is desired. For example, a thermally conductive molded body (e.g. polymer sheet) that is placed between electronic parts of a semiconductor device requires high thermal conductivity in the direction perpendicular to the sheet. Although conventional molded articles that have an excellent thermal conductivity in a certain direction contain graphite powder or carbon fibers, thermal conductivity of them are inadequate.
In the above-mentioned JP No.9-324127, die bond for a semiconductor element contains powdery graphite in thermosetting resin. The powdery graphite is dispersed randomly in the resin. Therefore, the molded article does not have good thermal conductivity in a certain direction.
An object of the present invention is to provide a thermally conductive molded article that has good thermal conductivity in a given direction and that is suitable for use as a heat radiation member, heat transfer member, or a component thereof in electronic hardware; and a method of making the molded article.